Operations, such as geophysical surveying, drilling, logging, well completing, and production, are typically performed to locate and gather valuable downhole fluids. Surveys are often performed using acquisition methodologies, such as seismic mapping to generate acoustic images of underground formations. These formations are often analyzed to determine the presence of subterranean assets, such as valuable fluids or minerals, or to determine if the formations have characteristics suitable for storing fluids. Although the subterranean assets are not limited to hydrocarbons such as oil, throughout this document, the terms “oilfield” and “oilfield operation” may be used interchangeably with the terms “field” and “field operation” to refer to a site where any types of valuable fluids or minerals can be found and the activities required to extract them. The terms may also refer to sites where substances are deposited or stored by injecting them into the surface using boreholes and the operations associated with this process.
During drilling and production operations, data is typically collected for analysis and/or monitoring of the operations. Such data may include, for instance, information regarding subterranean formations, equipment, and historical and/or other data.
Data concerning the subterranean formations is collected using a variety of sources. Such formation data may be static or dynamic. Static data relates to, for instance, formation structure and geological stratigraphy that define geological structures of the subterranean formation. Dynamic data relates to, for instance, fluids flowing through the geologic structures of the subterranean formation over time. Such static and/or dynamic data may be collected to learn more about the formations and the valuable assets contained therein.
Various equipment may be positioned about the field to monitor field parameters, to manipulate the operations and/or to separate and direct fluids from the wells. Surface equipment and completion equipment may also be used to inject fluids into reservoirs, either for storage or at strategic points to enhance production of the reservoir.
It is well known that mechanical disturbances can be used to establish acoustic waves in earth formations surrounding a wellbore, and the properties of these waves can be measured to obtain important information about the formations through which the waves have propagated. Properties of compressional, shear and Stoneley waves, such as their velocity (or its reciprocal, slowness) in the formation and in the wellbore, can provide useful information of formation characteristics that help in evaluation of the location and/or producibility of hydrocarbon resources.
Fractures in the earth play an essential role in influencing the movement of fluids in rocks. Fractures may be of natural origin or may be created artificially by operations associated with the exploitation of hydrocarbon or mineral reserves. For example, fractures may be deliberately induced in an oil or gas well by raising the wellbore fluid pressure until the surrounding rock fails in tension. Fractures produced in this manner are known as hydraulic fractures and they are frequently used in the petroleum industry to enhance production by providing high permeability conduits that promote the flow of hydrocarbons into the wellbore. The pressures measured in the wellbore during hydraulic fracturing may also be interpreted to obtain an estimate of the magnitude of stress in the earth.
In order to optimize production and correctly interpret stress in the earth, it is essential to understand the trajectories of fractures that intersect the wellbore. It is particularly important to determine whether such trajectories are planar or twisted. Twisted fractures tend to be less efficient producers than planar fractures as twisted fractures lead to tortuous flow paths and may contain choke points that impede the migration of hydrocarbons to the wellbore. Such fractures are sometimes produced during hydraulic fracturing operations in deviated (non-vertical) wells or more generally, in wells with trajectories that are not perpendicular to the smallest principal stress in the earth. For example, in locations where the vertical stress in the earth is the smallest principal stress, a hydraulic fracture in a vertical well may initiated as a vertical fracture and gradually twist to become horizontal as it propagates away from the wellbore.